This invention relates to the separation of CO.sub.2 from a scrubbing solution, and especially its application to the combined production of ammonia and urea.
In a more comprehensive aspect, this invention relates to the conversion of hydrocarbons to produce ammonia synthesis gas and CO.sub.2, by using a conventional steam reformer, downstream secondary reformer, and shift converter. The resultant ammonia synthesis gas, consisting essentially of hydrogen, nitrogen and carbon dioxide, is subjected to a high pressure scrubbing step with a physical solvent to remove acid impurities, and especially CO.sub.2 which is recovered from the loaded solvent. The regenerated solvent is recycled to the scrubbing step and the desorbed CO.sub.2 is used for urea synthesis.
As is known, for example, from DE-PS No. 27 21 462, ammonia, which is produced from a mixture of hydrogen and nitrogen, is used as a reactant for the synthesis of urea. The ammonia synthesis gas, consisting essentially of hydrogen, nitrogen and carbon dioxide, produced conventionally, is scrubbed to remove carbon dioxide. The carbon dioxide is then desorbed at ambient pressure from the loaded scrubbing solution in a downstream regeneration column and then is used for urea synthesis.
The above method has the drawback that the carbon dioxide must then be compressed from atmospheric pressure to the urea synthesis pressure of about 160 bar, for which a very great expenditure of energy is required. To avoid this disadvantage, it has already been proposed in patent application No. P 32 39 605.8 to reduce the pressure on the loaded solvent to an intermediate value and to heat the pressure-reduced solvent to partially degas the CO.sub.2. The degassed CO.sub.2 consequently accumulates under pressure and, after being cooled, can be used for urea synthesis. The partially regenerated solvent is subjected to a final pressureless regeneration stage. There the solvent is either again heated in a reboiler for the removal of the residual CO.sub.2 or freed of the residual CO.sub.2 by use of a vacuum.
In the first mentioned variant, there is however, a high steam requirement for the reboiler for the heating of the solvent in two stages--partial degassing and removal of residual CO.sub.2 as well for make-up heat exchange losses. In addition, cooling is required to compensate for the heating of the pumps and cooling of the crude gas. Linked with it is an expenditure for additional equipment such as a heat exchanger, reboiler, condenser with separator, and condensate pump.
Use of a vacuum on the other hand requires additional apparatus having a relatively high specific energy requirement. The attainable residual concentration of CO.sub.2 in the solvent is determined by the suction pressure of the vacuum compressor. Thus, the crude gas cannot be purified as well in the scrubber. The CO.sub.2 residual charge can be lowered by warming the solvent with waste heat, but then similar drawbacks occur as in the case of thermal regeneration.